How to batch convert between file formats?
Set the path variable and change the import line to whatever importer you are using :

import bpy
import os

path = 'C:/path/to/files/'  # set this path

for root, dirs, files in os.walk(path):
    for f in files:
        if f.endswith('.meshdata') :
            mesh_file = os.path.join(path, f)
            obj_file = os.path.splitext(mesh_file)[0] + ".obj"

            bpy.ops.object.select_all(action='SELECT')
            bpy.ops.object.delete()

            bpy.ops.import_scene.obj(filepath=mesh_file) # change this line

            bpy.ops.object.select_all(action='SELECT')

            bpy.ops.export_scene.obj(filepath=obj_file)